When I download the latest source code: aspnetwebstack-e0115a823029 , rebuild the system.web.mvc project and include it in my project to debug it I get the error that the WebPages 2.1 is used by this project, but my project references WebPages 2.0.

When debugging, can you open Debug -> Windows -> Modules, and sort the list by name, and verify that System.Web.Mvc.dll has symbols loaded? If not, right-click on the entry and select Load Symbols From -> Symbol Path.

I have http://srv.symbolsource.org/pdb/Public in my symbol path, among other things

The version is System.Web.Mvc.dllC:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Mvc\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Mvc.dllYesN/ACannot find or open the PDB file.
154.0.20710.07/10/2012 8:47 PM000007FEE6C40000-000007FEE6EC9000[2628] w3wp.exe: Managed (v4.0.30319)

When I right-click and select load symbols-> Microsft servers I get "C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Mvc\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Mvc.dll: Cannot find or open the PDB file." System.Web.dll loads
symbols just fine in the same manner.

I had exactly the same issue (correct version but no symbols), but after a day or so of trying everything I've been able to get it working. These are catch-all instructions that worked for me, they may contain unnecessary steps:

Remove and then reinstall MVC reference to your application with NuGet

Start debugging, open the modules window, and cross your fingers that ‘Symbols loaded’ is next to System.Web.MVC when it appears.

(sprinkle in restarts as you see fit, for me the key final step was reinstalling the NuGet reference)

My theory is that as the GAC library is used at runtime in preference to the local reference, the symbols are not found when they are searched for. By removing MVC from the GAC and then reinstalling the NuGet reference, it seems the local reference is used
and the symbols are found. It's counterintuitive as the System.Web.MVC.dll files are binary-equivalent between NuGet and the GAC, although they are dated differently.

I'm not totally convinced though as I tried to do the same thing for Razor and that didn't work.